tools/gn: Fix errors found by Facebook's flint tool.

tools/gn/ninja_binary_target_writer.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/ninja_binary_target_writer.h"
 
 #include <set>
 #include <sstream>
 
 #include "base/strings/string_util.h"
@@ skipping 29 matching lines @@
 
   void operator()(const std::string& s, std::ostream& out) const {
     out << " -D";
     EscapeStringToStream(out, s, options);
   }
 
   EscapeOptions options;
 };
 
 struct IncludeWriter {
-  IncludeWriter(PathOutput& path_output) : path_output_(path_output) {
+  explicit IncludeWriter(PathOutput& path_output) : path_output_(path_output) {
   }
   ~IncludeWriter() {
   }
 
   void operator()(const SourceDir& d, std::ostream& out) const {
     std::ostringstream path_out;
     path_output_.WriteDir(path_out, d, PathOutput::DIR_NO_LAST_SLASH);
     const std::string& path = path_out.str();
     if (path[0] == '"')
       out << " \"-I" << path.substr(1);
@@ skipping 411 matching lines @@
     return false;  // No tool for this file (it's a header file or something).
   const Tool* tool = target->toolchain()->GetTool(*computed_tool_type);
   if (!tool)
     return false;  // Tool does not apply for this toolchain.file.
 
   // Figure out what output(s) this compiler produces.
   SubstitutionWriter::ApplyListToCompilerAsOutputFile(
       target, source, tool->outputs(), outputs);
   return !outputs->empty();
 }